An electrophotographic technology has found widespread applications in the field of not only copying machines but also various printers and printing machines in recent years because it can provide an image of immediacy and high quality.
As for the photoreceptor which is the core of the electrophotographic technology, use of photoreceptors using organic photoconductive materials having advantages of retaining no pollution, ensuring easy film-forming, being easy to manufacture, and the like, has been the main stream in recent years instead of conventional inorganic photoconductive materials such as selenium, an arsenic-selenium alloy, cadmium sulfide or zinc oxide.
As the layer structure of the organic photoreceptor, there are known a so-called monolayer type photoreceptor obtained by dispersing a charge generation material in a binder resin, and a lamination type photoreceptor obtained by laminating a charge generation layer and a charge transport layer. The lamination type photoreceptor has been widely used because a stable high sensitivity photoreceptor can be provided by combining optimum layers of a charge generation material and a charge transport material each having a high efficiency, and characteristics are easily adjusted because of its wide material selection range. The monolayer type photoreceptor is slightly inferior to the lamination type photoreceptor in view of electric characteristics and its narrow material selection range, and accordingly has been is used to a limited extent. Further, the electrophotographic photoreceptor is repeatedly used in an electrophotographic process, i.e., in cycles of charging, exposure, development, transfer, cleaning, charge removal, and the like, during which it is subjected to various stress to be deteriorated. Among them, the chemical deterioration may be a damage to a photosensitive layer by strongly oxidizing ozone or NOx risen from, for example, a corona charger commonly used as a charger, and when repeatedly used, deterioration of electrical stability such as a reduction in the triboelectricity and an increase in the residual potential and accompanying image failure may occur. These are greatly due to chemical deterioration of a charge transport material contained in a large amount in the photosensitive layer.
Further, high sensitivity and high responsibility are required along with speeding up of the electrophotographic process in recent years. Among them, for high sensitivity, not only optimization of the charge generation material but also development of a charge transport material having favorable characteristics when combined with the charge generation material has been required, and for high responsibility, development of a is charge transport material having high mobility and showing a sufficiently low residual potential at the time of exposure has been required. As a charge transport material effective for high sensitivity and high responsibility, charge transport materials using various butadiene skeletons have been known (e.g. Patent Documents 1 to 3).
Patent Document 1: JP-A-60-175052
Patent Document 2: JP-A-10-312071
Patent Document 3: JP-A-2004-302032